Comonomer content and distribution (CCD) and/or short chain branching distribution (SCBD) is one of the key parameters controlling polyolefin properties. A precise and accurate CCD analysis is critical for new product development. Current test methods use crystallization based (Crystallization Elution Fractionation (CEF) and Temperature Rising Elution Fractionation (TREF)), or an interaction based technique (High Temperature Thermal Gradient Interaction Chromatography, HT-TGIC, or TGIC). However, these techniques have limited resolution and exhibit coelution and/or co-crystallization. The most challenging issue in CEF, and/or all the other crystallization based separation techniques, is co-crystallization. In order to minimize the amount of co-crystallization, a slow cooling process such as 1.5° C./hour is used at a cost of analysis time. Thus, there is a need for chromatography techniques that reduce co-crystallization, and improve resolution and accuracy, and decrease analysis time. Crystallization based separation techniques are described in the following references: Hazlitt, Journal of Applied Polymer Science: Applied Polymer Symposium 45, 25-37 (1990); Wild et al, Journal of Polymer Science, Poly. Phys. Ed., Vol. 20, p. 441 (1982)); EP2008701A1. JP 2013133451 discloses the use of narrow modulated temperature steps during cooling, about 5° C. per step, to improve the separation in temperature rising elution fractionation (TREF).
It was generally accepted that the crystallization process of TREF is largely controlled by the rate of crystal growth and the rate of nucleation. Wild et al. studied different cooling rates in TREF, and concluded that the support material (also commonly called column packing material) does not play a key role in TREF results (L Wild et al., Advances in Polymer Science 98, Springer-Verlag Berlin Heidelberg GmbH, P21). It had been widely accepted in the polyolefin industry, that glass, stainless shot, and crushed fire brick can be used as the column packing material, and these materials did not significantly change the final chromatography results.
However, as discussed above, there remains a need for chromatography techniques that reduce co-crystallization, and improve resolution and accuracy, and decrease analysis time. These needs have been met by the following invention.